APE6 preliminary review

Hooking into the registry is done in ``ccdproc`` at https://github.com/astropy/ccdproc/blob/master/ccdproc/ccddata.py#L368 with the functions that handling getting data into/out of a ``CCDData`` object just above that. Relevant docs are at http://ccdproc.readthedocs.org/en/latest/ccdproc/ccddata.html (look in the sections "Getting data in" and "Getting data out")

I think the current ``NDData``  doesn't actually tap into the I/O registry currently.